School Library Journal has an interview with Anthony Falcone about the Fair Use Project's involvement in the lawsuit

What’s the legal issue here?

It’s a critical, free-expression issue. The real issue it poses is the extent to which fans are going to have the right to participate in the content and fan community, and the extent to which they do so.

JKR and Co. say they are going to do their own definitive reference book. Is Vander Ark creating a whole new work or is he copying directly from Rowling's text? Did Rowling get the idea for a "lexicon" from Vander Ark? It will be interesting to see how this plays out.